1. Field of the Invention:
The invention relates to a vacuum-tight metal-ceramic soldered joint and a method for producing such joint.
2. Description of the Prior Art:
In the production of vacuum-tight metal-ceramic soldered joints, it is customary to provide the surface of the ceramic part to be soldered with a suitable metal coating, preferably applied by heat, to which the metal part can be soldered by means of soft or hard solder. Hard solders, for example, must be applied in all cases where the joint is required to have high mechanical strength and resistance to thermal stress.
Owing to the fact that the metals used have expansion coefficients differing widely from that of the ceramic, joints of this kind present problems. More particularly, high thermal stresses, which arise from the temperature difference between the soldering temperature and the room temperature of the joint, when in use, can lead to failure of the joint. These problems have hitherto been attempted to be solved by selecting materials having expansion coefficients as similar as possible for the metals to be soldered together and for the ceramic bonded to such materials. Also, by suitably designing the individual junction points, such problems can be avoided.
Metal alloys with a matched expansion coefficient have also been used together with a physical configuration of the seam which by its nature must have a diameter which is as small as possible. In addition to satisfactory wetting of the entire joint with solder through capillary action, these measures result in slight mechanical stress when thermal expansion occurs, owing to the higher expansion coefficient of the hard solder.